1. Field of the Invention
The present invention relates to novel highly porous catalyst materials in which the polarity of the interior surfaces can be purposefully adjusted. Said materials consist of mixed oxides which contain from 0.1 to 20% by weight of chemically bound organic residues. These organic residues serve to purposefully control the hydrophobicity of the materials and thus to improve the conversion, residence time and selectivity of heterogeneously catalyzed reactions.
2. Description of Related Art
An as yet unsolved problem concerns the polarity of the interior surfaces of heterogeneous catalysts which is hardly controllable. This surface polarity affects the interaction of the catalyst with the reactants, products and the solvent or other accompanying substances. With a good catalyst, this interaction is balanced in such a way that reactants and products will rapidly and effectively diffuse within the pore system of the catalyst whereas accompanying substances and side products are essentially kept out of the catalyst. Zeolites normally absorb water so well that they are widely used as desiccants. Therefore, before being used as catalysts, they must be dried most thoroughly at temperatures of up to 500.degree. C. There are exceptions, however. Thus, it is known that zeolites with a silicalite structure sorb very little water despite of their high interior surface area, which indicates a certain hydrophobicity (E. M. Flanigen, I. M. Bennett, R. W. Grose, J. P. Cohen, R. L. Polton, R. Kirchener, J. V. Smith, Nature 272 (1978), 437). This hydrophobicity is considered responsible for the special properties of Ti-containing silicalites (TS-1, TS-2) as selective catalysts (T. Tatsumi, K. Asano, K. Yanagisawa in Studies in Surface Science and Catalysis 84 (1994), 1861). While hydrophilic catalysts, such as microporous mixed-oxide glasses or Y zeolites, are not suitable for oxidation with H.sub.2 O.sub.2, the crystalline Ti silicalites, which are suitable for oxidation with H.sub.2 O.sub.2, are not capable of utilizing tert-butyl hydroperoxide (TBHP) as an oxidant.
The water absorbtivity of ZSM zeolites linearly depends on their Al content (D. H. Olsen, W. O. Haag, R. M. Lago, J. Catal. 61 (1980) 390). This hydrophobicity is the reason for the activity of Ti-containing silicalites (TS-1, TS-2) as selective oxidation catalysts with hydrogen peroxide as the oxidant. In contrast, the comparatively hydrophilic amorphous TiO.sub.2 /SiO.sub.2 materials of the same composition cannot utilize H.sub.2 O.sub.2 as an oxidant (Sheldon, J. Mol. Catal. 7 (1980), 107). With these materials, t-butyl hydroperoxide can be used as an oxidant for selective oxidations; however, these catalysts tend to be rapidly deactivated (R. A. Sheldon, J. A. Van Doorn, J. Catal. 31 (1973), 427). To date, silicalites have been the only heterogeneous catalysts which can use H.sub.2 O.sub.2 as an oxidant for selective oxidations under mild conditions. However, their hydrophobicity decreases as the Ti content increases, so that a contrary effect occurs as the fraction of catalytically active sites increases (Tatsumi et al., Stud. Surf. Sci. Catal. 84 (1994), 1861).
The particular importance of surface polarity also becomes evident in the use of zeolites in adsorption technology. Dealumination can produce hydrophobic zeolites which can be employed for adsorptive waste air cleaning and solvent recovery (Otten et al., Chem. Ing. Tech. 64 (1992), 915). The water absorption of other zeolites, however, cannot be suppressed as completely as that of the silicalites (Gunzel et al., Chem. Ing. Tech. 61 (1989), 66).
To conclude, very few zeolite structures can be rendered hydrophobic by dealumination, and thus, an extremely limited selection of materials are available for heterogeneous catalysis and adsorption in the presence of water under mild conditions. Moreover, dealumination is a comparatively cumbersome additional process step which mostly leads to a significant loss of structure, the formation of defect sites and a significant increase of undesirable amorphous fractions in the zeolite. In addition, the extent of dealumination cannot be purposefully adjusted, and the effect of a dealumination performed on the desired process must be established empirically. A known method for changing the surface polarity is subsequent modification. Thus, accessible surface hydroxy groups of catalysts can be subsequently modified by silylation with known silylating agents, such as (CH.sub.3).sub.3 SiCl or ((CH.sub.3).sub.3 Si).sub.2 NH (DE 23 11 822 C2), or other reagents. This subsequent modification has several drawbacks. The silylation in pores alters the pore geometry and thus possibly shape selectivities; hydroxy groups in micropores of &lt;1 nm can not or but incompletely be modified because of the bulky reagents; and the hydroxy groups thus silylated or alkylated are bound to the surface only through an O bridge to form a (CH.sub.3).sub.3 Si--O-- linkage or R--O-- linkage, and thus remain prone to hydrolysis. Therefore, there is a great need for novel highly porous materials in which the polarity of the surface can be purposefully adjusted in the preparation thereof independently of their elemental composition.